A traditional tower for a wind turbine is a tubular steel tower with tower sections placed on top of each other fixed to a concrete foundation. Due to transport and production restrictions, the external diameter has a limited size. Therefore, often the shell thickness is seen as the primary dimensioning parameter when increasing the tower height.
Simply increasing the tower shell thickness is however a very ineffective way of achieving increased bearing capacity and stiffness as these parameters only grow linear with the shell thickness. In comparison, increasing the diameter of the tower yields an increased bearing capacity with the diameter in the power of two, and a stiffness growing with the diameter in three.
To overcome such size limitation problem different solutions have been used. For instance, the bearing capacity of a tower such as a wind turbine tower can be increased and the stresses reduced in parts of the tower by installing a number of cables or wires from a number of anchors or foundations to attachment points on the tower. The cables may provide stability to the tower to reduce oscillations from wind and may reduce the loads in the part of the tower below the cables considerably. This so-called guyed or tethered tower will to some extent be de-loaded by the wire reactions and may thereby potentially be constructed by fewer long sections with a relatively smaller and thereby more transportable diameters.
Typically the wires are equally spaced around the tower, with one of their ends being attached to the tower, via e.g. eyes in plates mounted to the tower, and the other ends to anchor blocks at ground level. In e.g. U.S. Pat. No. 4,266,911 a separate ring is mounted around and movable along the tower and the wires may be mounted.
It is an object of the present invention to provide improvements to tethered wind turbine towers.